Any ophthalmic lens must meet a variety of criteria in order to be acceptable for wear. Foremost for a contact lens, any material placed over the cornea of the eye must in some way provide for the passage of oxygen to the eye and waste products away from the eye. With hydrated soft contact lenses this is accomplished by having a material that, inherent with its high water content (sometimes over 50%), passes oxygen to the eye via the water contained in the lens.
Hydrated soft contact lenses, however, can act as a wick, drawing water way from the tear fluid in the eye and hastening its evaporation. This results in the "dry eye" effect, wherein an excess of moisture is drawn away from the eye by the hydrophilic lens.
In contrast, the hard contact lens does not exhibit this wicking effect because water does not absorb and pass through the lens, but rather is underneath the lens. Hard lenses, however, can have a deleterious effect on the eye because of its non-pliable nature and the movement of the lens over the cornea whenever the wearer blinks can cause mechanical agitation.
Other desirable and undesirable characteristics are divided between hard and hydrated soft contact lenses.
For example, hard contact lenses do not absorb proteins and lipids to the extent that a high water content hydrogel does. The semi-rigid and hard lenses do adsorb some surface proteins and lipids, but these low water content materials absorb no proteins or lipids into the bulk material. Proteins and possibly lipids are taken into the material of the soft lenses along with the tear fluid where they may be deposited. In general, this necessitates cleaning of the hydrated lens to remove protein and lipid deposits. Furthermore, hard contact lenses typically exhibit a higher strength and higher refractive index because they contain more plastic and less water allowing them to be made thinner.
Soft hydrated contact lenses have enjoyed wide acceptance because of high degree of comfort and an extended period of wear. Most soft hydrophilic contact lens polymers produced over the last decade have strived to increase the water content of the material because of the water's contribution to wearer comfort and the passage of oxygen and carbon dioxide through the lens. This increase in water content, however, leads to the aforementioned problem with wicking of moisture away from the eye and also reduces the refractive index of the lens (i.e., the ability of the lens to bend light), and decreases the stiffness of the lens resulting in poorer handling properties. This in turn requires the lens to be thicker in order to meet the refractive requirements necessary for the optical correction needed by the wearer.
If a lens material is either not permeable enough to oxygen and carbon dioxide, or does not provide the "tear pumping" action required to move the tear layer between the cornea and the lens to transport oxygen and carbon dioxide, negative physiological responses occur, which include: acidosis, decreased metabolic rates, thinning of the cornea, microcysts, and stromal edema.
Other physiological problems can occur even with highly permeable lenses from effects such as protein deposits, lens ageing, occlusions, mechanical abrasion and bacterial contamination such as papillary conjunctivitis, acute inflammation, acute red eye, and 3 and 9 o'clock staining of the central cornea.
The importance of water content for oxygen permeability in a hydrogel contact lens is shown in FIG. 1. Permeability of a gas through a material is expressed as a quantitative value given by Dk, which is equal to the diffusion constant, D, times the solubility, k. At 35.degree. C., Dk for a typical hydrogel lens is quantitatively expressed as [2.0.times.10.sup.-11 ]e.sup.[.0442("%H20")] (cm.times.mm/s)(ml O.sub.2 /ml.times.mm Hg).
Despite the increased water content of hydrogel contact lenses, current hydrogel lens may not supply the cornea with enough oxygen and corneal edema during wear may not be as low as desired.
It is believed that extended wear contact lenses would at a minimum need to have a Dk/L (where L being the thickness of the lens) between 75.times.10.sup.-9 and 90.times.10.sup.-9 (cm.times.ml O.sub.2)/(s.times.ml.times.mm Hg) to reduce corneal edema to an acceptable level.
Current high water content lenses, for example, those that are approximately 70% water, need to be made at approximately 140 to 250 microns thickness to achieve the necessary optical and other physical properties. With this water content and at this thickness, it is seen in FIG. 2 that the Dk/L is about 55.times.10.sup.-9. Even with a hydrogel material having a water content of 80% and with a Dk equal to 53, a lens would have to be produced at approximately 70 microns in order for Dk/L to be 75.times.10.sup.-9.
As stated above, however, increasing the water content tends to lower the refractive index of the contact lens material and therefore require an increase in lens thickness. Even if this were not the case, however, thinner contact lenses have lower strength, less desirable handling properties and at high water content tend to dehydrate to such an extent that corneal staining occurs.
Examples of the current practice in the art of producing polymers for contact lenses is shown in European Patent Applications 0 330 614 and 0 330 615 both filed on Feb. 16, 1989. These publications describe contact lens polymers containing polyoxyalkylene and having the usual desirable properties of a soft contact lens, but both are described as containing in the hydrated state between 10% and 90% water, preferably between 35% and 55%, water by weight. European Patent Application number 0 263 061 filed on Aug. 24, 1987 also describes a contact lens material consisting of a polyoxyalkylene backbone unit and absorbing less than 10% water by weight. This polyoxyalkylene backbone forms a polymer which requires the addition of carbonyl containing monomers to induce surface wettability, but which also lowers oxygen permeability. EP patents 330614, 330615, and 330618 uses polyether and carbamate linkages to produce contact lens polymers of both low and high water content but also use small molecular weight monomers to increase the water content of the base polymer. These patents fail to teach the use of more biocompatible materials such as sugars which contain carbon atoms bonded to two oxygen atoms (geminal) as part of their structures. The materials of the references also require large amounts of hydrophilic modifiers to induce wettability and silicon materials require surface treatment of some type.
U.S. Pat. No. 3,225,012 discloses a polymer that is prepared by polymerizing 1,2: 5,6-di-0-isopropylidene-3-O-methacryloyl-D-glucose and then removing the isopropylidene groups from the glucose by acid hydrolysis. U.S. Pat. No. 3,356,652 describes a polymer that is derived from 2-(D-glucose)oxyethyl methacrylate. Both U.S. Pat. Nos. 3,225,012 and 3,356,652 use the glucose component of the polymer as a terminated pendant group off of a repeating carbon backbone, and not as the primary repeating group from which the polymer chain is formed.
It is an object of the invention to devise a contact lens construction and material where dehydration of the lens, and therefore the eye, is of little concern and furthermore does not allow proteins or other tear components to penetrate and deposit in the lens.
It is a further object of the invention, that a material and construction of a contact lens would have sufficient refractivity and a modulus of elasticity such that the lens could be processed thin enough to present a high degree of wearer comfort.
More specifically, it is the object of the invention to provide a contact lens material and construction wherein the permeability, Dk, of the material in combination with its thickness, L, would provide a gas transmissibility through the lens, equal to Dk/L, exceeding that attainable with current hydrogel soft contact lenses.
It is a further object of the invention to devise a polymer capable of inducing surface wettability without the addition of oxygen-permeability inhibiting carbonyl monomers.
The above objects of the invention are attained while maintaining the comfort level of current soft contact lenses by maintaining pliability and wettability of the lenses thus minimizing mechanical agitation of the cornea.